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Relation between Controller Cross Over Frequency and AC Sinusoidal Ref input freqency

Sahil
Expert 1585 points

Hi,

I am trying to understand what is the relation between a controller Cross over Frequency to a Variable AC Sinusoidal Reference input signal.

Say for instance I have a Bipolar Buck converter in H-Bridge fashion such that its control loop has a cross over frequency of 20kHz with a phase margin of 60 degree.

Now if I apply a variable ref input signal of sinusoidal wave of frequency Fref. How can I know at what freq my output will have an attenuation of -3dB ?

For an example, 

I have the following Bipolar Buck converter controller designed such that its crossover frequency is 20 kHz and phase margin of 60 degree. If I play a very slow Sinusoidal reference of 100Hz with 2.55 Vpk, I get exactly 2.55Vpk 100Hz output, however if I increase the Sinusoidal reference frequency to 1 kHz, I see the output distorted sinusoidal but attenuated more than half the amplitude a lot.

Alternatively if I ask the same question, what should I chose the crossover frequency of the controller to get my Sinusoidal Ref of 1 kHz attenuated to -3dB?

I thank you for your help.

Regards,

Iftikhar

  • 0
    TI__Guru 56825 points

     

    Typically, one would design the control loop to have a bandwidth of at least the target frequency, but there can be some complications.

    1) Typically loop response and bandwidths are measured in a "small signal" case with very small pertbations of the output voltage in the output or feedback, and from the reference voltage, sometimes converters have different loop performance characteristics relative to their reference input than their feedback path.

    2) Typically loop response bandwidths are measured with very small pertibations, ignoring limitations of current limit or slew-rate.

    3) Typically loop response bandwidth measurements measured with a constant input voltage, inducting a large, higher frequency oscillation on the output can induce an input oscillation as the source powering it has higher impedance.  If the control loop does not have some level of voltage feed-forward, the Input to Output bandwidth could be much lower than the feedback to output bandwidth.

    It would help to know more about the controller you are using, your design, and the source you are using.

    A 2.55Vpk-pk sign wave at 1kHz will require an output voltage slew-rate of 16mV/us, that will require 16A/mF output capacitance. 

    Is that hitting a current limit? 

    Is it causing the input voltage to droop and distorting the output voltage due to limited voltage feedforward? 

    Can the error amplifier drive the control loop fast and hard enough to track the reference?

    Are you able to share more details about your design, or application?